Adolescence is the period in which individuals are most likely to initiate the consumption of drugs of abuse. The onset of drug use during adolescence, however, also confers the highest risk of progressing from recreational use to addiction. This suggests that drug exposure during the critical window of adolescence may interfere with ongoing neuronal development and cause behavioral changes which facilitate subsequent substance abuse and addiction. However, there is still a significant lack of knowledge about the mechanisms accounting for these age-dependent effects of drug exposure. To address this gap in our knowledge, it is important to understand (1) the mechanism underlying the interaction between drugs of abuse and the adolescent brain, (2) how these interactions disrupt the development of neuronal circuitry, and (3) the resulting long-lasting changes in addiction-relevant behaviors. The netrin-1 guidance cue receptor DCC has recently emerged as a key molecular player in the adolescent organization of the mesocorticolimbic dopamine system, and expression of DCC is regulated by adolescent exposure to drugs of abuse. Exposure to the stimulant drug of abuse, amphetamine, during early adolescence disrupts the development of dopamine input to the medial prefrontal cortex of mice via alterations to DCC signaling. The long-term goal of the studies proposed here is to determine if amphetamine in adolescence induces the ectopic growth of mesolimbic dopamine axons to the medial prefrontal cortex by disrupting DCC-mediated guidance events, and to identify the enduring consequences on prefrontal cortex-dependent cognitive behavior. These neurobiological and behavioral alterations may underlie heightened vulnerability to progress to addiction in individuals that initiate drug use in adolescence. The experiments will employ neuroanatomical, gene transfection, and behavioral techniques in a mouse model to assess two specific aims. Aim 1 will determine whether amphetamine in adolescence increases prefrontal cortex dopamine innervation by promoting the ectopic growth of mesolimbic dopamine axons to this region. Aim 2 will assess the role of DCC-mediated netrin-1 signaling in the neuroanatomical and behavioral effects of amphetamine in adolescence. The results of the proposed research will move the field toward a clearer understanding of the mechanisms accounting for the age-dependent effects of drug exposure on addiction vulnerability. These results could be used to facilitate development of individualized prevention and treatment strategies aimed at adolescent drug users, who are at heightened risk to progress to addiction. !